Solid state lithium-iodine primary battery

ABSTRACT

A solid-state primary cell comprising a lithium anode, an iodine cathode containing a charge transfer complex and a solid lithium iodide electrolyte doped with a 1-normal-alkyl-pyridinium iodide. The anode surface can be coated with LiOH or Li 3  N. The iodine cathode comprises a complex of iodine and 1-normal-alkyl-pyridinium iodide and preferably contains titanium dioxide powder, alumina gel powder or silica gel powder admixed with the complex.

BACKGROUND OF THE INVENTION

This invention relates to solid-state primary batteries comprising alithium anode, an iodine cathode containing a charge transfer complexand a solid lithium iodide electrolyte doped with a1-normal-alkyl-pyridinium iodide.

Many of electronic circuits presently used require a power supply whichprovides an output of high voltage but low current. Generally compactbatteries reliably operable for a prolonged period of time are desirablefor fulfilling such a requirement.

However, usual primary cells incorporating a liquid electrolyte haveserious drawbacks. For example, the liquid electrolyte is liable toleak. This is a fatal drawback when the cell is used in a system inwhich the leakage of the electrolyte is not permissible if slightest.Another drawback is that they need a separator. The separator used incompact cells will cause internal short-circuiting when broken, whilereducing the interior space for accommodating the active cellcomponents.

Efforts have been made to overcome these drawbacks. For instance,research has been directed to the development of non-liquid type cellsin which all the cell elements are in solid state. Among the cells ofthis type heretofore proposed, those with a lithium anode are featuredby a high energy density.

The performance of solid-state cells depends largely on the ionicconductivity of the solid electrolyte used. Thus solid-state cells willhave a high internal resistance and therefore deliver low output currentwhen containing a solid electrolyte which has such a low conductivitythat the electron conductivity of the cathode mixture of the cell is atleast 100 to 1000 times as high as the conductivity of the electrolyte.

Liang et al., J. Electrochemical Soc., 123,453 (1976), have proposed asolid-state cell comprising a lithium anode, a cathode mixture of leadiodide and lead sulfide and a solid lithium iodide electrolyte dopedwith alumina. The solid lithium iodide electrolyte doped with aluminahas a relatively high ionic conductivity of about 10⁻⁵ ohm⁻¹.cm⁻¹ at 25°C., whereas the discharge reaction product of the cell is lithium iodidehaving a relatively low ionic conductivity of about 10⁻⁷ ohm⁻¹.cm⁻¹ at25° C.

Schneider et al., J. Power Sources 5,651 (1975), have proposed asolid-state cell comprising a lithium anode, a cathode mixture composedof a charge transfer complex of iodine with poly-2-vinylpyridinecontaining an excess of iodine, and a solid lithium iodide electrolyte.The electrolyte and discharge reaction product of the cell are bothlithium iodide having a relatively low ionic conductivity of about 10⁻⁷ohm⁻¹.cm⁻¹ at 25° C. The cathode mixture which has a relatively lowelectron conductivity of about 10⁻⁴ ohm⁻¹.cm⁻¹ at 25° C. is fully usefulas such since the electrolyte used has a still lower conductivity of10⁻⁷ ohm⁻¹.cm⁻¹. The cellnevertheless has the drawback of being unableto deliver a relatively high output current because of the lowconductivity of the electrolyte and, moreover, remains to be improved inits shelf life. In fact, when the battery is stored at hightemperatures, iodine diffuses markedly from the cathode mixture throughthe solid electrolyte, possibly draining the cell due to the attendantinternal short-circuiting during storage.

SUMMARY OF THE INVENTION

An object of this invention is to provide an improved solid-state cellwhich has a lithium anode and in which lithium iodide doped with a1-normal-alkyl-pyridinium iodide having a relatively high ionicconductivity is the solid electrolyte and cell discharge reactionproduct.

Another object of this invention is to provide an improved solid-statecathode mixture containing iodine and having a relatively high electronconductivity.

Another object of this invention is to provide a solid-state cell whichhas a lithium anode coated with LiOH or Li₃ N and which is therebyrendered less susceptible to internal self-discharge.

The cell of this invention comprises a lithium anode, a solidelectron-conductive iodine-containing cathode and a solid lithium iodideelectrolyte doped with a 1-alkyl-pyridinium iodide.

The cell reaction is represented by the following equation:

    C.sub.5 H.sub.5 NRI.sub.n+(n- 1)Li→C.sub.5 H.sub.5 NRI(n-1)LiI

wherein n is the number of iodine atoms. Since lithium has the smallestelectrochemical equivalent and is the most electronegative metal, theelectrochemical system has a high energy density of about 425 mWh/cc.The electrolyte resulting from the cell discharge reaction is lithiumiodide doped with a 1-normal-alkyl-pyridinium iodide, namely C₅ H₅NR(n-1)LiI. At 25° C., this lithium salt has an ionic conductivity of10⁻⁴ ohm⁻¹.cm⁻¹ which is much higher than the conductivity, about 10⁻⁷ohm⁻¹.cm⁻¹, of lithium iodide. This electrolyte is formed by the directsurface-to-surface contact of the anode with the cathode. Thus thelithium anode reacts with the charge transfer complex of this invention,forming a solid electrolyte interconnecting the anode and the cathodeand comprising lithium iodide doped with the 1-normal-alkyl-pyridiniumiodide.

The solid cathode mixture comprises a charge transfer complex of iodinewith a 1-normal-alkyl-pyridinium iodide and an iodine-inert electricallynonconductive powder admixed with the complex. Examples of preferablepowders are titanium dioxide powder, alumina gel powder and silica gelpowder. Generally the cathode mixture is held in contact with a currentcollector preferably of carbon or metal which is inert to the cathodemixture.

Similarly an inert current collector is used in the usual manner for thesoft anode to provide a terminal for electrical connection. The lithiumanode may preferably be coated with lithium hydroxide or lithium nitrideto mitigate the internal self-discharge of the cell which graduallyoccurs during storage due to the diffusion of iodine through theelectrolyte layer.

The corrosion of the cathode current collector metal by the cathodecomplex also involves spontaneous internal self-discharge of the cell.When made of super ferrite stainless steel containing at least 30 wt. %of Cr and at least 2 wt. % of Mo, the current collector is highlyresistant to corrosion.

Other features and advantages of this invention will become moreapparent from the following detailed description with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIGS. 1a to 1c show conductivities at 25° C. of some solid electrolytesof lithium iodide doped with 1-normal-alkyl-pyridinium iodides;

FIG. 2 is a view in vertical section showing a button-shaped cell whichis an embodiment of this invention;

FIG. 3a is a view in vertical section showing a flat-shaped cell whichis another embodiment of this invention;

FIG. 3b is a plan view partly broken away and showing the same;

FIGS. 4a to 4e are diagrams illustrating the states of charge transfercomplexes of iodine and 1-normal alkyl-pyridinium iodides useful in thisinvention;

FIG. 5 is a diagram showing the discharge curves of the cells preparedin Example 1 as determined by the discharge of constant current of 100μA at -15° C.;

FIG. 6 is a diagram showing the discharge curves of the cells preparedin Example 2 as determined by the discharge of constant current of 2 μAat -15° C.;

FIG. 7 is a diagram showing the discharge curves of cells prepared inExample 2 as determined by the discharge of constant current of 2 μA at50° C.;

FIG. 8 is a diagram showing the discharge curves of the cells preparedin Example 3 as determined by the discharge of constant current of 100μA at 25° C.; and

FIGS. 9a and 9b are diagrams showing veriations in the internalresistance of the cells prepared in Examples 4 and 5 as determined whenthe cells were stored at 60° C. for a prolonged period of time.

DETAILED DESCRIPTION OF THE INVENTION

Briefly, the main object of this invention is to alleviate the problemof high internal resistance attributable to the solid electrolyte andthe attendant problem of low output current and to provide novel cells.

We have found that the conductivity of lithium iodide can be greatlyimproved by the addition of 1-normal-alkyl-pyridinium iodides whichwould produce defects in the solid lithium iodide electrolyte matrix.FIG. 1a shows conductivities of lithium iodide doped with1-methyl-pyridinium iodide (C₅ H₅ NCH₃ I(n-1)LiI), FIG. 1b shows thoseof lithium iodide doped with 1-normal-propyl-pyridinium iodide (C₅ H₅NC₃ H₇ I(n-1)LiI), and FIG. 1c shows those of lithium iodide doped with1-normal-hexyl-pyridinium iodide (C₅ H₅ NC₆ H₁₃ I(n-l)LiI). In thesediagrams, the conductivity is plotted as ordinate vs. the number ofiodine atoms, n, as abscissa. As will be apparent from FIGS. 1a to 1c,these lithium salts have the highest conductivities when the n value isbetween 6 and 12. The conductivity increases with the increase in thesize of the pyridinium ion added.

The ratio of ionic radii r⁺ /r⁻ is theoretically about 0.28. This valueindicates that the coordination number of Li⁺ cation relative to I⁻anion is 4, but since the lithium iodide crystal is actually of thesodium chloride type, the coordination number is 6.

The difference in the coordination number between the theoretical valueand the actual value means that the apparent radius of Li⁺ cation isgreater than is theoretically thought. This results in the unexpectedlyhigh ionic conductivity of lithium iodide, about 10⁻⁷ ohm⁻¹.cm⁻¹.

This is also the case with the addition of a large organic cation tolithium iodide. The addition of a large organic cation such as thepyridinium ion of this invention expands the crystal lattice of lithiumiodide, with the coordination number increasing from 6 to 8. This alsomeans an increase in the apparent radius of Li⁺ cation, leading to anincrease in the conductivity of the corresponding lithium salt.

Examples of 1-alkyl-pyridinium iodides useful as additives for dopinglithium iodide are preferably 1-normal-propyl-pyridinium iodide,1-normal-butyl-pyridinium iodide, 1-normal-pentyl-pyridinium iodide and1-normal-hexyl-pyridinium iodide. These additives give highconductivities of at least 10⁻⁵ ohm⁻¹.cm⁻¹ over a wide range of nvalues. The corresponding lithium salts have a melting point higher than60° C. which is the upper limit of temperatures at which cells areusually used.

FIG. 2 is a sectional view showing a button-shaped cell embodying thisinvention and having a thickness of 2.5 mm and desired diameter.

FIGS. 3a and 3b are a sectional view and a plan view showing aflat-shaped cell which is another embodiment of this invention andmeasuring 1.5 mm in thickness, 52 mm in length and 25 mm in width.

The button-shaped cell comprises a 0.2-mm-thick lithium anode 1 havingone side in contact with an anode current collector 3 made of metal andserving also as a closure plate and the other side in contact with aniodine-containing cathode mixture 2, a thin layer 5 of solid lithiumiodide electrolyte formed on the surface of the lithium anode 1 by thecontact of the lithium anode 1 with the cathode mixture 2 and doped witha 1-normal-alkyl-pyridinium iodide, a cathode current collector 4serving also as the shell of the cell and made of super ferritestainless steel containing at least 30 wt. % of Cr and at least 2 wt. %of Mo, and a plastics insulator 6 for electrically insulating thecathode current collector 4 from the anode current collector 3. Theflat-shaped cell includes the same components as above and furthercomprises a plastics sheet 7 sealing the assembly of these components onthe four sides as by heat adhesion. The lithium anode 1 which is usuallyprepared from a sheet or foil may alternatively be formed on the currentcollector 3 by vacuum evaporation, electrode-position or some otherusual method. Since the cell of this invention is affected by the watercontained in the atmosphere, it is assembled and sealed in a dry box orthe like which is maintained at a relative humidity of up to 2% with useof a drying agent such as P₂ O₅.

When assembling the cell, it is critical to avoid contact between theanode current collector and the cathode mixture which would causeinternal short-circuiting. Insofar as is presently known, theelectronically conductive material which, when brought into directcontact with the iodine cathode mixture of this invention, forms anionically conductive, electronically non-conductive film on the surfaceof the material exposed to the cathode mixture is limited to lithiummetal, while the other conductive materials usable for the anode currentcollector are all electronically connected to the cathode mixture whenbrought into contact therewith. To avoid such connection in the cellsshown in FIGS. 2, 3a and 3b, the anode current collector is fullycovered with the lithium anode, the outer periphery of which is heldbetween the plastics insulator and the anode current collector to keepthe cathode mixture out of contact with the anode current collector.

It is also critical that the material used for the cathode currentcollector be resistant to iodine contained in the cathode mixtureaccording to this invention and having high corrosive effect on metals.To select suitable materials for the cathode current collector, weimmersed metal materials in a charge transfer complex C₅ H₅ NC₂ H₅ I₁₅at 60° C. for a prolonged period and tested them for corrosionresistance in terms of the resulting weight reduction (wt. %). The metalmaterials tested were iron, SUS304 stainless steel, titanium and Fe(66wt. %)-Cr(31 wt. %)-MO(3 wt. %) and Fe(68 wt. %)-Cr(30 wt. %)-Mo(2 wt.%) super ferrite stainless steels. Table 1 shows the results, whichindicate that the super ferrite stainless steels containing at least 30wt. % of Cr and at least 2 wt. % of Mo are most resistant to corrosion.

                  TABLE 1                                                         ______________________________________                                        Test       Weight reduction (wt. %)                                           Specimen   In 50 days  In 100 days                                                                             In 150 days                                  ______________________________________                                        Fe         disappeared disappeared                                                                             disappeared                                  SUS 304    21          41        63                                           Ti         16          30        38                                           Fe(66)--Cr(31)--                                                                         3           3.5       3.5                                          Mo(3)                                                                         Fe(68)--Cr(30)--                                                                         4           4         4                                            Mo(2)                                                                         ______________________________________                                    

To select suitable insulators for electrically insulating the cathodecurrent collector from the anode current collector, we immersed variousplastic materials in charge transfer complex C₅ H₅ NC₂ H₅ I₁₅ at 60° C.for a long period and measured the time dependence of resistivity of theplastic films. Insulators are required to have high resistivity ifcontacting with a charge transfer complex. The plastics tested werepolystyrene, polypropylene, polyester, polyvinylchlorid, polyethylene,polyimide and copolymer of ethylene and tetrafluoroethylene.

Table 2 shows the results, which indicate that copolymer of ethylene andtetrafluoroethylene, polypropylene, polyethylene and polyimide aresuitable plastics for insulator.

                  TABLE 2                                                         ______________________________________                                        Resistivity ohm . cm                                                          Test    Before    After      After   After                                    Specimen                                                                              immersing 10 days    30 days 100 days                                 ______________________________________                                        Poly-   10.sup.16 disappeared                                                                              disappeared                                                                           disappeared                              styrene                                                                       Poly-   10.sup.14 broken     disappeared                                                                           disappeared                              ester                                                                         Polyvinyl-                                                                            10.sup.15 10.sup.11  10.sup.10                                                                             10.sup.10                                chlorid                                                                       Poly-   10.sup.15 10.sup.14  10.sup.14                                                                             10.sup.14                                ethylene                                                                      Poly-   10.sup.15 10.sup.14  10.sup.14                                                                             10.sup.14                                propylene                                                                     Polyimide                                                                             10.sup.17 10.sup.16  10.sup.16                                                                             10.sup.16                                Complex of                                                                            10.sup.17 10.sup.17  10.sup.17                                                                             10.sup.17                                E & TFE                                                                       ______________________________________                                    

FIGS. 4a to 4e are diagrams showing the states of some charge transfercomplexes useful as cathode mixtures according to this invention.

It is well known that the charge transfer complex is a substanceconsisting of two elements, namely an electron acceptor and an electrondoner, and having a higher electron conductivity.

The charge transfer complexes suitable for use in this invention areionic complexes composed of iodine ion serving as the electron doner andiodine serving as the electron acceptor.

The charge transfer complexes of this invention have an electronconductivity of at least about 10⁻² ohm⁻¹.cm⁻¹ at room temperature whenin a liquid state and also when present in both liquid and solid states.They have an electron conductivity of about 10⁻⁸ ohm⁻¹.cm⁻¹ when in asolid state. Such a complex is readily available by mixing togetheriodine and a 1-normal-alkyl-pyridinium iodide in the dry box alreadymentioned.

The cathode mixture, which is brought into direct contact with thelithium anode when assembling the battery, must be made completely freefrom any electron conductive material other than the charge transfercomplex. Since the charge transfer complex solidifies and has a reducedelectron conductivity at lower temperatures, the temperature at whichthe cell is usable is limited by the temperature at which the complexcompletely solidifies. To obtain cells which are usable at temperatureswhich are much lower than 0° C., for example -15° C., it is preferableto use as the electron donor of the charge transfer complex a1-normal-alkyl-pyridinium iodide in which the alkyl has at least 3carbon chains as will be apparent from FIGS. 4a to 4e.

The cathode mixture is composed of the above-mentioned charge transfercomplex in a liquid state and having a high electronic conductivity andan iodine-inert electrically nonconductive powder preferably of titaniumdioxide, alumina gel or silica gel admixed with the complex, the mixturebeing in the form of a solid powder having a high electronicconductivity. The proportion of the inert nonconconductive powderrelative to iodine is so determined that the cathode mixture ispress-moldable to pellets which effectively retain the liquid chargetransfer complex against flowing out and which have an electronicconductivity of at least 10⁻² ohm⁻¹.cm⁻¹.

Various pellets were press-molded from iodine, 1-normal-butyl-pyridiniumiodide and silica gel powder up to 10 μm in particle size and adaptedfor use in chromatographic analysis. Table 3 shows the bulk densities(d; gr/cc at 25° C.) of the pelletized cathode mixture capable ofpositively retaining the charge transfer complex and the conductivities(σ_(e), ohm⁻¹. cm⁻¹) thereof.

                                      TABLE 3                                     __________________________________________________________________________    SiO.sub.2 gel                                                                         n in C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.n                        content (wt. %)                                                                       3     5     6     9     12    15    27                                __________________________________________________________________________    d       2.6   3.0   2.8   2.6   2.8   3.2   3.4                               σ.sub.e                                                                         1.1 × 10.sup.-2                                                               1.9 × 10.sup.-2                                                               2.1 × 10.sup.-2                                                               3.8 × 10.sup.-2                                                               1.9 × 10.sup.-2                                                               1.5 × 10.sup.-2                                                               4.2 × 10.sup.-3             d       2.4   2.8   2.6   2.5   2.7   3.0   3.1                               σ.sub.e                                                                           1 × 10.sup.-2                                                               1.8 × 10.sup.-2                                                               1.9 × 10.sup.-2                                                               3.5 × 10.sup.-2                                                               1.6 × 10.sup.-2                                                               1.5 × 10.sup.-2                                                               2.0 × 10.sup.-3             d       2.3   2.6   2.5   2.3   2.5   2.8   3.0                               σ.sub.e                                                                           8 × 10.sup.-3                                                               1.1 × 10.sup.-2                                                               1.6 × 10.sup.-2                                                               2.0 × 10.sup.-2                                                               1.4 × 10.sup.- 2                                                                1 × 10.sup.-2                                                               1.5 × 10.sup.-3             d       2.2   2.5   2.3   2.1   2.3   2.5   2.8                               σ.sub.e                                                                         2.6 × 10.sup.-3                                                                 9 × 10.sup.-3                                                               1.3 × 10.sup.-2                                                               1.8 × 10.sup.-2                                                                 9 × 10.sup.-3                                                               8.5 × 10.sup.-3                                                                 9 × 10.sup.-4             __________________________________________________________________________

The voltage of the cell of this invention linearly lowers with the lapseof time when the cell is discharged at a constant current. Generally thecurve representing the voltage-time relation during the discharge of thecell is given by:

    V.sub.T =Vemf-{Ro-i/s+R(i/s).sup.2 t}

wherein V_(T) is the voltage of the cell, Vemf is the electromotiveforce of the cell, i is the discharge current value, s is the area ofthe lithium anode, Ro is the internal resistance value of the cell dueto the growth of electrolyte layer during storage of the cell and R isthe internal resistance value resulting from the formation ofelectrolyte due to the discharge.

The value Ro depends on the square root of storage time as is typical ofthe diffusion limit reaction. The increase in Ro can be inhibitedperfectly or partially by covering the surface of the lithium anode witha lithium hydroxide layer or lithium nitride layer. Since the lithiumhydroxide or lithium nitride remains thermodynamically relatively stableagainst the attack by the iodine of the cathode, such a layer inhibitsthe diffusion of iodine from the cathode. What is more advantageous isthat the thin layer of lithium nitride which is highly lithium-ionconductive will produce no adverse effect on the cell during discharge.Lithium hydroxide, although nonconductive for lithium ions, will coatthe lithium anode in the form of a porous thin layer and furthergradually react with the iodine from the cathode to form lithium iodidemonohydrate which is more lithium-ion conductive than lithium iodide.Similarly, therefore, lithium hydroxide will in no way act adversely onthe cell during discharge.

The cells of this invention will be described below in greater detailwith reference to examples.

EXAMPLE 1

Cells incorporating a lithium anode and a solid electrolyte of theconstruction shown in FIGS. 3a and 3b were assembled with use of fourkinds of cathode mixtures, namely a mixture of C₅ H₅ N.C₄ H₉ I₉ or C₅ H₅N.C₄ H₉ I₅₀ charge transfer complex and silica gel powder up to 10 μm inparticle size and adapted for use in chromatographic analysis, a mixtureof C₅ H₅ N.C₄ H₉ I₉ and alumina gel powder up to 10 μm in particle sizeand adapted for use in chromatogrphic analysis, a mixture of C₅ H₅ N.C₄H₉ I₉ and titanium dioxide powder up to 10 μm in particle size, and apoly-2-vinylpyridine charge transfer complex having 10 iodine atoms pernitrogen atom. The group C₄ H₉ is a normal-butyl, i.e. normal--C₄ H₉.

With reference to FIGS. 3a and 3b, each of the cells was assembled withuse of a 0.1-mm-thick cathode current collector 4 made of super ferritestainless steel containing at least 30 wt. % of Cr and at least 2 wt. %of Mo. A piece of polypropylene nonwoven fabric, 1 mm in thickness, 22mm in width and 50 mm in length and serving to insulate the cathodecollector from the anode, was placed over the collector 4, with theabove-mentioned cathode mixture placed on the fabric over an area,smaller than the surface area of the fabric, of 18 mm in width and 40 mmin length. The assembly was molded at pressure of 0.2 t/cm² to form acathode mixture layer of about 1 mm in thickness. An anode currentcollector 3 supporting a lithium anode having a smaller area than thenonwoven fabric and measuring 20 mm in width, 45 mm in length and 0.2 mmin thickness was thereafter superposed on the molded cathode layer. Theassembly was sealed with about 0.1 -mm-thick polypropylene film toprepare a flat-shaped cell.

Table 4 shows the electromotive force (O.C.V.) of each of the cells thusprepared, and the internal resistance of the cell measured after storingthe cell for 3 months at 25° C. One month after the preparation, thecell was tested at -15° C. for discharge at a constant current of 100μA. FIG. 5 showing the discharge curve thus obtained reveals that thecells of this invention can deliver a greater amount of current than theconventional cell in which the electrolyte is lithium iodide and thecathode mixture is a charge transfer complex of iodine andpoly-2-vinylpryidine.

                                      TABLE 4                                     __________________________________________________________________________                                    Internal resistance (Ω)                 Cell No.                                                                           Cathode mixture    O.C.V. (volts)                                                                        after 3 months at 25° C.               __________________________________________________________________________    1    C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.9 + SiO.sub.2 gel (17 wt.             %)                 2.79    250                                           2    C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.9 + Al.sub.2 O.sub.3 gel (20          wt. %)             2.80    320                                           3    C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.9 + TiO.sub.2 (25 wt.                                    2.79    290                                           4    C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.50 + SiO.sub.2 gel (5 wt.                                2.80    1800                                          5    poly-2-vinylpyridine I.sub.10                                                                    2.81    1750                                          __________________________________________________________________________

EXAMPLE 2

Cells, 11.6 mm in diameter and 2.5 mm in thickness, of the constructionshown in FIG. 2 and comprising a lithium anode and a solid electrolytewere prepared with use of a cathode mixture of C₅ H₅ N-normal-RI₉(R:CH₃, C₂ H₅, C₃ H₇, C₄ H₉, C₆ H₁₃ or C₈ H₁₇) charge transfer complexand silica gel powder up to 10 μm in particle size and adapted for usein chromatographic analysis. The cathode mixture was prepared by mixingtogether the specified amount of C₅ H₅ N-normal-RI₉ powder, iodinepowder and silica gel powder, and molding the mixture into pellets of8.6 mm in diameter and about 1.7 mm in thickness at pressure of 0.2ton/cm². The pellets were placed on a 0.3-mm-thick stainless steelclosure plate serving also as an anode current collector 3 and providedwith a 0.2-mm-thick lithium sheet 1 and a polypropylene insulator 6 asshown in FIG. 2. A 0.3 -mm-thick cathode current collector 4 serving asthe shell of the cell and made of stainless steel containing at least 30wt. % of Cr and at least 2 wt. % of Mo was placed over the resultingassembly. The outer periphery of the cathode current collector 4 wasthen crimped at pressure of 2 tons/cm² to complete the cell.

Table 5 shows the electromotive force (O.C.V.) of each of thebutton-shaped cells thus prepared, and the internal resistance of thecell measured after storing the cell for 3 months at 25° C. One monthafter the preparation, the cells were tested at -15° C. for discharge ata constant current of 2 μA to obtain the discharge curves shown in FIG.6 and were similarly tested at 50° C. to obtain the discharge curvesshown in FIG. 7. FIGS. 6 and 7 indicate that the cells in which thecharge transfer complexes having alkyl of 3 to 6 carbon chains are usedas the cathode mixture exhibit outstanding discharge performance over awide temperature range.

                                      TABLE 5                                     __________________________________________________________________________                                    Internal resistance (kΩ)                Cell No.                                                                           Cathode mixture    O.C.V. (volts)                                                                        after 3 months at 25° C.               __________________________________________________________________________    6    C.sub.5 H.sub.5 N.CH.sub.3 I.sub.9 + SiO.sub.2 gel (2 wt.                                        2.80    12                                            7    C.sub.5 H.sub.5 N.C.sub.2 H.sub.5 I.sub.9 + SiO.sub.2 gel (17 wt.                                2.79.sub.5                                                                            8.3                                           8    C.sub.5 H.sub.5 N.C.sub.3 H.sub.7 I.sub.9 + SiO.sub.2 gel (17 wt.                                2.79.sub.5                                                                            4.1                                           9    C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.9 + SiO.sub.2 gel (17 wt.                                2.79    4.2                                           10   C.sub.5 H.sub.5 N.C.sub.6 H.sub.13 I.sub.9 + SiO.sub.2 gel (17 wt.            %)                 2.79    3.9                                           11   C.sub.5 H.sub.5 N.C.sub.8 H.sub.17 I.sub.9 + SiO.sub.2 gel (17 wt.            %)                 2.78.sub.5                                                                            3.6                                           __________________________________________________________________________

EXAMPLE 3

Cells, 23 mm in diameter and 2.5 mm in thickness, of the constructionshown in FIG. 2 and comprising a lithium anode and a solid electrolytewere prepared with use of a cathode mixture of C₅ H₅ N-normal-C₄ H₉I_(n) (n: 3, 5, 6, 9, 12, 27 or 50) charge transfer complex and silicagel powder up to 10 μm in particle size and adapted for use inchromatographic analysis. The cells were prepared in the same manner asin Example 2 except that the cathode mixture was molded into pellets of18 mm in diameter and about 1.7 mm in thickness.

Table 6 shows the electromotive force (O.C.V.) of each of thebutton-shaped cells thus prepared, and the internal resistance of thecell measured after storing the cell for 3 months at 25° C. One monthafter the preparation, the cells were tested at 25° C. for discharge ata constant current of 100 μA to obtain the discharge curves shown inFIG. 8. The results indicate that the cells in which the charge transfercomplexes of this invention have an n value of 6 to 12 exhibitoutstanding discharge performance, as can be expected from FIG. 1showing ionic conductivities of lithium iodide doped with1-alkyl-pyridinium iodides.

                                      TABLE 6                                     __________________________________________________________________________                                    Internal resistance (Ω)                 Cell No.                                                                           Cathode mixture    O.C.V. (volts)                                                                        after 3 months at 25° C.               __________________________________________________________________________    12   C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.3 + SiO.sub.2 gel (10 wt.             %)                 2.46    3520                                          13   C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.5 + SiO.sub.2 gel (15 wt.                                2.75    1050                                          14   C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.6 + SiO.sub.2 gel (15 wt.                                2.77     810                                          15   C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.9 + SiO.sub.2 gel (17 wt.                                2.79     790                                          16   C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.12 + SiO.sub.2 gel (20 wt.            %)                 2.80     850                                          17   C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.27 + SiO.sub.2 gel (15 wt.            %)                 2.80.sub.5                                                                            1980                                          18   C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.50 + SiO.sub.2 gel (5 wt.                                2.80    5200                                          __________________________________________________________________________

EXAMPLE 4

The increase in the internal resistance Ro of cells during the storageparticularly at high temperatures is attributable to the growth of theelectrolyte during the storage and is one of the factors governing thedischarge voltage of the cell. The increase of the resistance Ro can beeffectively inhibited by coating the lithium anode with a thin layer oflithium hydroxide over the anode surface to be brought into contact withthe cathode mixture, although this requires an additional procedure whenassembling the cell.

Cells, 11.6 mm in diameter and 2.5 mm in thickness, of the constructionshown in FIG. 2 were prepared which incorporated a lithium anode coatedwith lithium hydroxide and a solid electrolyte.

The lithium hydroxide layer was formed by placing a lithium anode 1 inthe recess of an anode current collector 3 serving also as a closureplate and fitting a plastics insulator 6 over the peripheral portion ofthe assembly to obtain an anode module, and allowing the anode module tostand for about 0.5 minute in a closed container maintained at arelative humidity of 11% at 20° C. with a saturated aqueous solution oflithium chloride. Except for this procedure, the cells were made in thesame manner as in Example 2. Table 7 shows the electromotive force(O.C.V.) of the button-shaped cells thus prepared. FIG. 9a showsvariations in the internal resistance of the cells during storage at 60°C., as determined from the cell voltage drop resulting from theapplication of direct current for 1 to 2 seconds.

                                      TABLE 7                                     __________________________________________________________________________                            Exposure to                                           Cell No.                                                                           Cathode mixture    H.sub.2 O vapor (time in min.)                                                             O.C.V. (volts)                           __________________________________________________________________________    19   C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.12 + SiO.sub.2 gel (20 wt.            %)                 0            2.80                                     20   C.sub.5 H.sub.5 N.C.sub.3 H.sub.7 I.sub.9 + SiO.sub.2 gel (17 wt.                                0.5          2.86                                     21   C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.12 + SiO.sub.2 gel (20 wt.            %)                 0.5          2.91                                     22   C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.9 + SiO.sub.2 gel (17 wt.                                0.5          2.87                                     23   C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.6 + SiO.sub.2 gel (15 wt.                                0.5          2.90                                     24   C.sub.5 H.sub.5 N.C.sub.6 H.sub.13 I.sub.9 + SiO.sub.2 gel (17 wt.            %)                 0.5          2.85                                     __________________________________________________________________________

EXAMPLE 5

The increase in the internal resistance of cells during storage at hightemperatures can be effectively inhibited by coating the lithium anodesurface with a thin layer of lithium nitride instead of lithiumhydroxide.

Cells, 11.6 mm in diameter and 2.5 mm in thickness, of the constructionshown in FIG. 2 were made which included a lithium anode coated withlithium nitride and a solid electrolyte.

The lithium nitride layer was formed by allowing the same anode moduleas used in Example 4 to stand for about 45 minutes in a dry containermaintained at a temperature of 60° C. with its interior air replacedwith nitrogen gas of high purity (99.99%) supplied at a steady rate.Except for this procedure, the cells were made in the same manner as inExample 2. Table 8 shows the electromotive force (O.C.V.) of thebutton-shaped cells thus obtained. FIG. 9b shows variations in theinternal resistance of the cells during storage at 60° C., as determinedfrom the cell voltage drop resulting from the application of directcurrent for 1 to 2 seconds.

                                      TABLE 8                                     __________________________________________________________________________                            Exposure to                                           Cell No.                                                                           Cathode mixture    N.sub.2 gas (time in min.)                                                               O.C.V. (volts)                             __________________________________________________________________________    25   C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.12 + SiO.sub.2 gel (20 wt.            %)                  0         2.80                                       26   C.sub.5 H.sub.5 N.C.sub.3 H.sub.7 I.sub.9 + SiO.sub.2 gel (17 wt.                                45         2.75                                       27   C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.12 + SiO.sub.2 gel (20 wt.            %)                 45         2.76                                       28   C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.9 + SiO.sub.2 gel (17 wt.                                45         2.78                                       29   C.sub.5 H.sub.5 N.C.sub.4 H.sub.9 I.sub.6 + SiO.sub.2 gel (15 wt.                                45         2.78                                       30   C.sub.5 H.sub.5 N.C.sub.6 H.sub.13 I.sub.9 + SiO.sub.2 gel (17 wt.            %)                 45         2.72                                       __________________________________________________________________________

What we claim is:
 1. A substantially anhydrous cell comprising a solidlithium anode, a solid .[.electronically.]. .Iadd.electrically.Iaddend.conductive iodine cathode containing a charge transfer complexof iodine with a 1-normal-alkyl-pyridinium iodide .Iadd.and aniodine-inert electrically nonconductive powder, .Iaddend.and a solidelectrolyte of lithium iodide doped with the 1-normal-alkyl-pyridiniumiodide.
 2. A cell according to claim 1 in which the anode surface iscoated with lithium hydroxide.
 3. A cell according to claim 1 in whichthe anode surface is coated with lithium nitride.
 4. A cell according toclaim 1, 2 or 3 in which .Iadd.the .Iaddend.1-normal-alkyl-pyridiniumiodide has an alkyl group of at least three normal carbon chains.
 5. Acell according to claim 1, 2 or 3 in which the charge transfer complexcontains between 6 and 12 atoms of iodine for each atom of nitrogen. 6.A cell according to claim 1, 2 or 3 in which the .[.cathode containsan.]. iodine-inert electrically nonconductive powder .Iadd.is.Iaddend.selected from the group consisting of titanium dioxide powder,alumina gel powder, and silica gel powder.
 7. A cell according to claim5 in which the cathode is a mixture consisting of the charge transfercomplex and .Iadd.a .Iaddend.silica gel powder.
 8. A cell according toclaim 7 in which the charge transfer complex has 9 atoms of iodine foreach atom of nitrogen of 1-normal-butyl-pyridinium iodide.
 9. A cellaccording to claim 7 in which the cathode contains between 10 and 20weight percent of the silica gel powder.
 10. A cell according to claim1, 2 or 3 in which the iodine cathode is separated from an anode currentcollector.
 11. A cell according to claim 1, 2 or 3 in which a cathodecurrent collector is made of a super ferrite stainless steel consistingof at least 30 weight percent of Cr, at least 2 weight percent of Mo,and Fe. .[.12. A cell according to claim 1, 2 or 3 in which an insulatoris made of iodine resistant plastics selected from the group consistingof copolymer of ethylene and tetrafluoroethylene, polypropylene,polyethylene and polyimide..].
 13. A process for assembling a cellaccording to claim 1, 2 or 3 in which the lithium anode surface isdirectly attached to the iodine cathode.
 14. A process for assembling acell according to claim 1, 2 or 3 in which the lithium anode surface isdirectly pressed against the iodine cathode.